Many modern machines have a relatively broad range of available transmission gear ratios. Given such flexibility in operation, wide variance may exist in transmission output shaft speed across the range of available gears and engine speeds. The driveshaft typically rotates in concert with the transmission output shaft and may therefore typically be operated in a similarly broad range of speeds. While many machine powertrain systems are intended to have broad flexibility, transmission and engine designs do not always account for certain hardware limitations of the machine, potentially leading to undesired stresses and strains during operation, and even failure of the equipment.
For instance, certain movable components of the powertrain, and in particular drive line components such as the driveshaft, may under certain operating conditions rotate at speeds in excess of the intended design. In other words, a given transmission gear ratio may be selected such that the transmission output speed, i.e. a driveshaft speed, at relatively high engine speeds may be greater than a predetermined speed limit for the driveshaft. In some instances, overspeeding of the driveshaft can cause it to orbit, resonantly vibrating. Under such conditions, the driveshaft can exert radial force on the transmission tailshaft which, if imparted at an appropriate frequency, may cause a resonant vibration in the engine/transmission package, and potentially damage the powertrain.
Engineers have addressed driveshaft and related overspeed problems in a variety of ways. One approach, commonly used in conventional passenger vehicles has been to design the powertrain components with sufficient robustness and mass moment that high driveshaft speeds, and associated resonant driveshaft vibrations, are not problematic. While this approach has been successful in some instances, other types of machines have powertrain hardware less readily modified.
In relatively large on-highway trucks, for example, a common approach to driveshaft overspeed problems has been to “gear bind” the transmission. In a typical gear binding strategy, certain transmission gears are made unavailable under all conditions, either by mechanically blocking their engagement in some way, or by electronically turning the gears off. In other words, even where the transmission has numerous theoretically available gears, the operator and/or control system is permanently prevented from engaging certain gears which have the potential to result in driveshaft speeds in excess of driveshaft speed limits.
While gear binding can have the intended effect of generally preventing driveshaft overspeed, it presents significant problems. Among these is the fact that it is inherently wasteful and cost ineffective to build a transmission with certain gears that are never used.
Another shortcoming of gear binding strategies relates to fuel economy. In order to operate a machine, such as an on-highway truck, at relatively higher speeds appropriate for on-highway travel, the unavailability of certain higher gears requires the machine to be operated at relatively high engine speeds to attain a desired ground speed. It is well known in the art that continually operating an engine toward the upper end of its speed range is characterized by less than optimum fuel economy. Nevertheless, it is common for operators to “redline” the machine for long periods of time, resulting in poor fuel economy because of the inability to switch into a higher gear that would provide for operation at relatively lower engine speeds. A driveshaft overspeed prevention approach is thus desirable which would allow the use of higher transmission gears when desired, but still provide a robust means of ensuring the driveshaft will not be driven at excessive speeds, and risk associated damage to powertrain components.
The present disclosure is directed to one or more of the problems or shortcomings set forth above.